1. Field of the Invention
This invention relates generally to the regulation of the voltage and current characteristics of electrical circuits and, more particularly, to circuits providing a reference (or bias) voltage and/or providing a constant current source that are substantially independent of the power sources activating the circuit. The present invention uses enhanced mode FET devices to provide voltage and current control while being coupled to relatively high voltage supplies.
2. Description of the Related Art
Referring now to FIG. 1A, a prior art technique for providing a reference voltage is illustrated. The cathode of a Zener diode D1 is coupled to the positive voltage supply +V.sub.S. The anode of the Zener diode D1 is coupled through resistor R1 to the cathode of Zener diode D2. The anode of Zener diode D2 is coupled to the negative voltage supply -V.sub.S. The characteristics of a Zener diode, reverse biased, are such that the voltage across the Zener diode is essentially independent of the current flowing therethrough. Thus, a bias voltage relative to the positive supply terminal, V.sub.BIAS+, and a bias voltage relative to the negative supply, V.sub.BIAS-, are maintained that are relatively independent of fluctuations in the voltage supplies, the changes in the supply voltages being compensated for in the resistor R1. Because the voltage across a Zener diode is independent of the current therethrough only to a first approximation, improvements have been sought to increase the stability of the V.sub.BIAS+ and V.sub.BIAS- bias voltages relative to fluctuations in the supply voltages.
The stability of the bias voltages can be improved, according to the prior art, by the circuit shown in FIG. 1B. In FIG. 1B, the resistor R1 is replaced by the n-channel junction FET device Q1 with a resistor R2 in the gate-source circuit, the gate of FET device Q1 being coupled to the cathode of Zener diode D2. The FET device/resistor combination forms a constant current source which, when combined with the constant voltage characteristics of the Zener diodes, provides increased stability for the bias voltages. Despite the increased stability of the bias voltage V.sub.BIAS+ and V.sub.BIAS-, greater stability of the bias voltages against parameter changes, such as fluctuations in supply voltage levels, has been sought. In addition, the depletion mode transistor have limited voltage handling capacity and, at the present time, are relatively expensive.
A need has therefore been felt for apparatus and method for increasing the stability of bias voltages capable of implementation with high voltage supplies that do not have the disadvantages of the prior art circuits and do not require depletion mode transistor devices.